The invention pertains in general to a communications receiver with a dynamic signal-to-noise ratio (SNR) requirement and, more particularly, to a system and method for optimizing the precision of datapath modules within a communications receiver with a dynamic SNR requirement.
In a multi-carrier communications system a plurality of sub-carriers are transmitted over a single transmission path at the same time. Orthogonal frequency division multiplexing (OFDM) is one commonly implemented multi-carrier scheme in which the plurality of sub-carriers are orthogonal in frequency. The sub-carriers in an OFDM carrier signal typically overlap in frequency but do not interfere with each other. Each sub-carrier may be modulated using any one of a number of modulation schemes, including quadrature amplitude modulation (QAM) (e.g., 16 and 64-QAM) and quadrature phase-shift keying (QPSK).
An OFDM transceiver typically applies a fast Fourier transform (FFT) to separate sub-carriers of a received OFDM carrier signal. In a similar manner, an OFDM transceiver typically applies an inverse FFT (IFFT) to generate an OFDM carrier signal for transmission by combining a plurality of sub-carriers. The FFT, in general, defines a set of processes for reducing the time required to compute a discrete Fourier transform (DFT) and has several different implementations. However, each implementation of the FFT performs the DFT over a finite duration sequence of N sample points, x(n), that represent samples of a received OFDM carrier signal. The DFT may be defined as:
            X      ⁡              (        k        )              =                  ∑                  n          =          0                          N          -          1                    ⁢                          ⁢                        x          ⁡                      (            n            )                          ⁢                  ⅇ                                    -                              j                ⁡                                  (                                                            2                      ⁢                      π                                        N                                    )                                                      ⁢            nk                                          k      =      0        ,    1    ,                  …        ⁢                                  ⁢        N            -      1      
The DFT is inherently a block process that is computed over a block of N samples; N being typically defined as the number of sub-carriers transmitted over the received OFDM carrier signal. The N samples, x(n), are multiplied successively by complex exponentials over the range of frequencies of the sub-carriers, and each product is summed. The output of the above equation, x(k), represents the value of the spectrum for the kth frequency, i.e., the value of the sub-carrier at the kth frequency. In this way, sub-carriers of a received signal may be separated using a DFT computation.
An OFDM receiver generally implements a dedicated FFT unit that performs the above calculations using data represented in a fixed-point format. Samples of a received signal, x(n), are represented in a fixed-point format along with results of the FFT unit. The precision of a fixed-point number is determined by the number of bits used in its representation. As a result, in any finite length fixed-point data representation, some quantization noise may be introduced into the system. In an OFDM receiver, it is important that the quantization noise, introduced within the FFT unit, does not dominate the overall signal-to-noise ratio (SNR) requirement of the receiver. Typical OFDM receivers implement an FFT unit with enough precision to handle the worst case SNR requirement.
Although an FFT unit must be designed to handle the worst case SNR requirement of an OFDM receiver, in many cases the instantaneous SNR requirement is substantially less. For example, an OFDM receiver may be required to handle a worst case SNR requirement of 30 dB and, as a result, fixed-point data sizes within the FFT unit must be large enough so that quantization noise does not prevent the receiver from achieving this worst case SNR requirement. However, since the SNR requirement of the OFDM receiver may be dynamic, at any given point in operation, the OFDM receiver may have an instantaneous SNR requirement below 30 dB. As a result, the FFT unit operates with excess precision in many instances. The datapath modules within the FFT unit consume excess power performing computations and operations on fixed-point data bits that are unnecessary. Since power consumption is often critical in OFDM receivers that may be dependent on batteries, any excess power consumed decreases the utility of these devices.
Therefore, what is needed is a system and method for dynamically adjusting the precision of datapath modules within an FFT unit without adversely affecting the reception and demodulation of an OFDM carrier signal.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.